Crustal Dynamics: Unified Understanding of Geodynamics
Processes at Different Time and Length Scales
35.5˚
N

σ1

5mm/yr2000

35˚

San-in Shear Zone
1927
1943
1925
2016
Ref.

135.5˚

135˚134.5˚134˚133.5˚

133˚132.5˚

34.5˚Editorial Board (September 2019)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

Vice Editors-in-Chief
Masato Furuya, Hokkaido University, Japan
Solid Earth Science

Nozomu Nishitani, Nagoya University, Japan
Space Science

Editors
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Hauke Hussmann
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Alexey Kuvshinov
Juanjo Ledo
Huixin Liu
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Mitsuhiro Nakagawa
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Earth, Planets and Space
Iio et al. Earth, Planets and Space (2018) 70:97
https://doi.org/10.1186/s40623-018-0869-6

Open Access

PREFACE

Crustal dynamics: unified understandingof geodynamic processes at different timeand length scales
Yoshihisa Iio1*, Richard H. Sibson2, Toru Takeshita3, Takeshi Sagiya4, Bunichiro Shibazaki5
and Junichi Nakajima6

Keywords: Crustal dynamics, Tohoku-oki earthquake, Stress, Strength, Deformation, Fault zone, Friction, Rheology,
Crustal fluid, Materials sciencedrop. Modeling the full spatial distribution of static
stress changes during the mainshock is probably neededto account for the spatial complexity of coseismic stressrotations. Otsubo et al. (2018) employed the multiple
inverse method (MIM) to demonstrate significant variations in the normal faulting stress state around Iwaki
City, Japan, over a period of a few years prior to the 2011
Mw 9.0 Tohoku megathrust earthquake. Such variations in stress state require a low differential stress state
which they attribute to overpressured fluids in the focal
regions. Terakawa (2017) analyzed slip plane diversity
within microearthquake swarms around Mt. Ontake stratovolcano to make the case that local swarm activity isdriven by regions where pore fluids are overpressured by
10–30 MPa. Matsumoto and Shigematsu (2018) reported
measurements of fault zone permeability from boreholeintercepts along the Median Tectonic Line (MTL) in Mie
Prefecture, SW Japan, finding values more than 100–
700 times the permeability of the surrounding protolithassemblage of crystalline rocks. While reported permeabilities (5 × 10−16 m2 > k > 3 × 10−19 m2) are generally
too high to contain overpressured fault fluids at depth,
it has to be kept in mind that the measurements weremade under low confining pressure at depths of only a
few hundred meters. Sibson (2017) advanced the hypothesis that the local attainment of the tensile overpressurestate (Pf > σ3) is associated with the formation and activation of fault–fracture meshes distributed throughout
tabular volumes. Except in the near surface this generally
requires near-lithostatic fluid overpressures. Interlinkageof shear fractures with fluid-saturated extension fractures

The 2011 Tohoku-oki earthquake occurred on an unexpected scale, which made us realize that the generation mechanisms of earthquakes are poorly understood.
To provide a unified view of the geodynamic processes
including earthquake generation processes in the Japanese arc–trench system, it is necessary to clarify theabsolute values of crustal stresses, the stress–strain field,
and the basic properties of the island arc crust and mantle, in particular, those of fault zones. This special issue
includes 22 papers, and they are divided into several categories: (1) crustal stress and overpressured fluids, (2)stress and rupture heterogeneity, (3) large-scale deformation in the Japanese Island Arc, (4) shear zone detectedby Satellite Geodesy, (5) rheology of crustal fault zones,(6) materials science of rock deformation.
Several papers analyzed spatiotemporal changes incrustal stress related to earthquake generation and volcanic eruption and discussed involvement of overpressured fluids in seismogenesis and fault zone properties
related to overpressure. Hardebeck (2017) investigatedcoseismic and postseismic rotations of principal stressaxes caused by three M > 8.8 subduction megathrust
ruptures. The largest coseismic stress rotations occur
just above the Moho depth of the overriding plate wherelarge continuous slip patches appear (from seismologicalstudies) to coincide with areas of intense fluid overpressuring inferred to promote near-complete shear stress*Correspondence: iio@rcep.dpri.kyoto-u.ac.jp
1
Research Center for Earthquake Prediction, Disaster Prevention ResearchInstitute, Kyoto University, Uji, Japan
Full list of author information is available at the end of the article© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

1

Crustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales
Iio et al. Earth, Planets and Space (2018) 70:97

Page 2 of 4

denudation history of the region. Distinct morphostructural provinces defined by apatite He ages are distinguished, the Abukuma Mountains on the fore-arc
side (64.3–49.6 Ma), the Ou Backbone Range along
the volcanic front (11.4–1.5 Ma), and the Asahi Mountains on the back-arc side (< 10 Ma). Denudation ratesof < 0.1 mm/year are estimated for the Abukama Mountains, 0.1–1.0 mm/year for the Ou Backbone range, and
0.1–0.3 mm/year for the Asahi Mountains. These techniques could be extended across other segments of thearc, but possible thermal effects of magmatism need tobe carefully considered.
Finer-scale deformation is estimated by Satellite
Geodesy along major fault zones in Japan. Nishimura
and Takada (2017) used GNSS velocity data to define
the San-in dextral shear zone with a width of c. 50 km
accommodating c. 5 mm/year of dextral shearing along
the northern coastline of southwest Japan. Major recent
earthquake ruptures appear to follow anticipated trajectories of conjugate Riedel shears within the shear zone.
Takada et al. (2018) employed Satellite Geodesy (InSAR
and GNSS) to define a sharp velocity gradient across the
Ushikubi fault within the dextral Atotsugawa fault system
in central Japan. Analysis of InSAR data shows interseismic deformation to be spatially heterogeneous within thestrain concentration zone.
Rheology of large-scale crustal fault zones is essential
in the crustal dynamics, because it can control deformation in the whole crust in island arcs. Nakajima and Matsuzawa (2017) used high-quality waveform data from adense seismic network to explore the three-dimensional
P-wave attenuation structure at depth along the Niigata–Kobe Tectonic Zone (NKTZ) in central Japan. The
study confirms spatial relationships between attenuation
structures and surface deformation along the NKTZ.
Anelastically weakened lower crust west of the Itoigawa–Shizuoka Tectonic Line (ISTL) promotes surface contraction over a region about 100 km wide while anelastic
deformation in the thick, shallow sedimentary basin eastof the ISTL restricts surface deformation to a narrow
region (25–40 km). These observations account qualitatively for regional variations in the width of the highstrain-rate zone across the ISTL, placing constraints on
the character of deformation in the subsurface. Dojo andHiramatsu (2017) used the spatial distribution of coda Q
from the analysis of waveform data to investigate a highstrain-rate region in the northeastern part of the Niigata–
Kobe Tectonic Zone (NKTZ). Coda Q in the 2–3 Hz
frequency band correlates spatially with S wave velocity
at 25 km depth, while Coda Q in the 4–8 Hz band correlates with S wave velocity perturbations at 10 km depth.
Results indicate that a combination of deformation in theupper crust as well as ductile deformation in the lower

slows slip transfer, allowing such mesh structures tofunction as rheological units incorporating viscous dashpots capable of giving rise to a variety of anomalous slowslip phenomena.
Full understanding of crustal dynamics requires clarification of stress and strength heterogeneities, and rupture heterogeneity. Yukutake and Iio (2017) conducted aprecise analysis of hypocenters and focal mechanisms ofupper crustal aftershocks from the 2000 Mw 6.6 Western
Tottori, Japan, earthquake which involved predominantly
sinistral strike-slip along a NNW–SSE fault structure disrupted by a conjugate set of dextral cross-faults. Aftershocks around the mainshock rupture plane occur withina tabular zone 1.0–1.5 km thick, significantly broader
than the likely damage zone, with diverse mechanisms.
The aftershocks apparently represent rupture of fractures
surrounding the mainshock rupture rather than reshearof the primary rupture, caused by stress changes arisingfrom heterogeneous slip distribution along the mainshock rupture. Iio et al. (2017) employed a high-density
seismological network in western Nagano Prefecture.
Focal mechanisms were inverted to show that the crustal
stress field can generally be regarded as uniform at a scaleof 1 km throughout the study region, but that strengthis heterogeneous, varying over comparatively short distances (~ 100 m). Ando et al. (2017) analyzed complex
patterns in the wave radiation and surface displacement of the 2014 Mw 6.2 northern Nagano earthquake
sequence which involved predominantly reverse slip onan irregularly segmented rupture. Observations include
foreshock occurrence, large differences between the firstmotion focal mechanisms and the CMT, and along-strikevariations in surface displacement. Aftershocks reveal
a more complex geometry in the northern half of the
focal area, correlated with along-strike variation of fault
activity and maturity. Dynamic rupture simulations tookaccount of the observationally determined regional stressfield and fault geometry. The observed complexity is
explained as the effect of non-planar fault geometry witha number of branch faults and bends. Maeda et al. (2018)
explored the spatial relationship between upper crustal
structure and seismicity in the Kii Peninsula of southwest
Japan, where the stress field and the predominant focal
mechanism change with depth. They attribute this stressheterogeneity to localized thermal stress from a buried
heat source in the lower crust.
Deformation occurs in response to stress and/or
stress changes, and it reflects material properties where
the deformation occurs. Thus, it is crucial to measure
deformation and/or deformation rate in the JapaneseIsland Arc. Sueoka et al. (2017) employed (U-Th)/He
thermochronometric analyses across southern Tohoku
in the Japan arc to reconstruct the long-term uplift and2Earth, Planets and Space
Iio et al. Earth, Planets and Space (2018) 70:97

Page 3 of 4

crust may contribute to the high strain rate in the northeastern NKTZ. Zhang and Sagiya (2017) modeled strain
concentration in two dimensions within the lower crust,assuming steady fault sliding in the upper crust andductile flow in the lower crust according to laboratoryderived power-law rheology with a yield threshold at thebrittle–ductile transition. Possible physical mechanisms
for strain concentration in the lower crust are investigated including frictional and shear heating, grain size,and power-law creep, taking account also of the role ofwater in promoting crystal plasticity.
Materials science of rock deformation is fundamental
for not only the crustal dynamics but also all the studies
in solid earth sciences. Consequently, its progress is crucial for our understanding of crustal dynamics. Fukuda
et al. (2018) showed how the addition of small amountsof water to polycrystalline anorthite under high temperature induces a change from distributed fracturing to plastic flow promoting grain-size-sensitive creep in the lower
middle crust. Kameda et al. (2017) investigated the alteration and dehydration of subducting oceanic crust, specifically pillow basalts within the Shimanto belt showing
how the saponite–chlorite conversion within mixed layer
C/S minerals may contribute fluid to plate boundary fault
systems with consequent mechanical effects. Kuwatani
and Toriumi (2017) employed a new forward modelingtechnique for analyzing retrogressive hydration reactions.
Results indicate that changes in mineral composition are
mainly controlled by pressure and temperature, but thatchanges in mineral modes are controlled by the degreeof water infiltration. Matsumura et al. (2017) statisticallyanalyzed two probability density functions to evaluate a
microboudin paleopiezometer applied to stretched tourmaline grains within Archean metacherts from the East
Pilbara Terrane in Western Australia. They found that
an elastic matrix model is preferable to a Newtonian viscous model for analyzing the stresses involved in microboudinage of columnar tourmaline grains within thequartz matrix of the metamorphic tectonite. Tsubokawa
and Ishikawa (2017) reported on the preparation of submicron polycrystalline olivine and clinopyroxene by sintering. Incorporation of trace amounts of graphite allows
experimental investigations into the influence of graphite
on mantle rheology and seismic velocity.

and Earthquake Engineering, Building Research Institute, Tsukuba, Japan.
Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology, Tokyo, Japan.

6

Acknowledgements
We express our sincere gratitude to the authors who contributed to thisspecial issue and the reviewers who evaluated the contributions and gavethoughtful comments and suggestions.
Competing interests
The authors declare that they have no competing interest.
Funding
This study was partly supported by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan, under the Earthquake and Volcano
Hazards Observation and Research Program, and KAKENHI Grant Numbers
26109001-26109007, and 15K21755.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Received: 5 June 2018 Accepted: 6 June 2018

References
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propagation on geometrically complex fault with along-strike variationof fault maturity: insights from the 2014 Northern Nagano earthquake.
Earth Planets Space 69:130. https://doi.org/10.1186/s40623-017-0715-2
Dojo M, Hiramatsu Y (2017) Spatial variation in coda Q in the northeasternpart of Niigata–Kobe Tectonic Zone, central Japan: implication of thecause of a high strain rate zone. Earth Planets Space 69:76. https://doi.
org/10.1186/s40623-017-0663-x
Fukuda J, Muto J, Nagahama H (2018) Strain localization and fabric development in polycrystalline anorthite + melt by water diffusion in anaxial deformation experiment. Earth Planets Space 70:3. https://doi.
org/10.1186/s40623-017-0776-2
Hardebeck JL (2017) The spatial distribution of earthquake stress rotations
following large subduction zone earthquakes. Earth Planets Space 69:69.
https://doi.org/10.1186/s40623-017-0654-y
Iio Y, Yoneda I, Sawada M, Miura T, Katao H, Takada Y, Omura K, Horiuchi S(2017) Which is heterogeneous, stress or strength? An estimation fromhigh-density seismic observations. Earth Planets Space 69:144. https://doi.org/10.1186/s40623-017-0730-3
Kameda J, Inoue S, Tanikawa W, Yamaguchi A, Hamada Y, Hashimoto Y,
Kimura G (2017) Alteration and dehydration of subducting oceanic crust
within subduction zones: implications for décollement step-down andplate-boundary seismogenesis. Earth Planets Space 69:52. https://doi.
org/10.1186/s40623-017-0635-1
Kuwatani T, Toriumi M (2017) Thermodynamic forward modeling of retrogressive hydration reactions induced by geofluid infiltration. Earth Planets
Space 69:18. https://doi.org/10.1186/s40623-017-0607-5
Maeda S, Matsuzawa T, Toda S, Yoshida K, Katao H (2018) Complex microseismic activity and depth-dependent stress field changes in Wakayama,
southwestern Japan. Earth Planets Space 70:21. https://doi.org/10.1186/
s40623-018-0788-6
Matsumoto N, Shigematsu N (2018) In-situ permeability of fault zones
estimated by hydraulic tests and continuous groundwater-pressure
observations. Earth Planets Space 70:13. https://doi.org/10.1186/s4062
3-017-0765-5
Matsumura T, Kuwatani T, Masuda T (2017) Statistical model selection between
elastic and Newtonian viscous matrix models for the microboudinpalaeopiezometer. Earth Planets Space 69:83. https://doi.org/10.1186/
s40623-017-0669-4
Nakajima J, Matsuzawa T (2017) Anelastic properties beneath the Niigata–
Kobe Tectonic Zone, Japan. Earth Planets Space 69:33. https://doi.
org/10.1186/s40623-017-0619-1

Authors’ contributions
All authors of this article are guest editors for this special issue. All authors readand approved the final manuscript.
Author details
1
Research Center for Earthquake Prediction, Disaster Prevention ResearchInstitute, Kyoto University, Uji, Japan. 2 Department of Geology, Universityof Otago, Dunedin 9054, New Zealand. 3 Department of Natural History Sciences, Graduate School of Sciences, Hokkaido University, Sapporo 060-0810,
Japan. 4 Disaster Mitigation Research Center, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8601, Japan. 5 International Institute of Seismology

3Crustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales
Iio et al. Earth, Planets and Space (2018) 70:97

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Nishimura T, Takada Y (2017) San-in shear zone in southwest Japan,
revealed by GNSS observations. Earth Planets Space 69:85. https://doi.
org/10.1186/s40623-017-0673-8
Otsubo M, Miyakawa A, Imanishi K (2018) Normal-faulting stress state associated with low differential stress in an overriding plate in northeast Japanprior to the 2011 Mw 9.0 Tohoku earthquake. Earth Planets Space 70:51.
https://doi.org/10.1186/s40623-018-0813-9
Sibson RH (2017) Tensile overpressure compartments on low-angle thrustfaults. Earth Planets Space 69:113. https://doi.org/10.1186/s4062
3-017-0699-y
Sueoka S, Tagami T, Kohn BP (2017) First report of (U–Th)/He thermochronometric data across Northeast Japan Arc: implications for the long-term
inelastic deformation. Earth Planets Space 69:79. https://doi.org/10.1186/
s40623-017-0661-z
Takada Y, Sagiya T, Nishimura T (2018) Interseismic crustal deformation in andaround the Atotsugawa fault system, central Japan, detected by InSAR
and GNSS. Earth Planets Space 70:32. https://doi.org/10.1186/s4062
3-018-0801-0

Terakawa T (2017) Overpressurized fluids drive microseismic swarm activity
around Mt. Ontake volcano, Japan. Earth Planets Space 69:87. https://doi.
org/10.1186/s40623-017-0671-x
Tsubokawa Y, Ishikawa M (2017) Sintering polycrystalline olivine and polycrystalline clinopyroxene containing trace amount of graphite from natural
crystals. Earth Planets Space 69:128. https://doi.org/10.1186/s4062
3-017-0717-0
Yukutake Y, Iio Y (2017) Why do aftershocks occur? Relationship between
mainshock rupture and aftershock sequence based on highly resolved
hypocenter and focal mechanism distributions. Earth Planets Space
69:68. https://doi.org/10.1186/s40623-017-0650-2
Zhang X, Sagiya T (2017) Shear strain concentration mechanism in thelower crust below an intraplate strike-slip fault based on rheologicallaws of rocks. Earth Planets Space 69:82. https://doi.org/10.1186/s4062
3-017-0668-5

4

Earth, Planets and SpaceFRONTIER LETTER

Open Access

The spatial distribution of earthquake stress rotations
following large subduction zone earthquakes
Jeanne L. Hardebeck
Earth, Planets and Space 2017, 69:69 DOI:10.1186/s40623-017-0654-y
Received: 27 January 2017, Accepted: 12 May 2017, Published: 18 May 2017

Abstract
Rotations of the principal stress axes due to great subduction zone earthquakes have been used to infer low differential stressand near-complete stress drop. The spatial distribution of coseismic and postseismic stress rotation as a function of depth andalong-strike distance is explored for three recent M ≥ 8.8 subduction megathrust earthquakes. In the down-dip direction, thelargest coseismic stress rotations are found just above the Moho depth of the overriding plate. This zone has been identified
as hosting large patches of large slip in great earthquakes, based on the lack of high-frequency radiated energy. The large
continuous slip patches may facilitate near-complete stress drop. There is seismological evidence for high fluid pressures inthe subducted slab around the Moho depth of the overriding plate, suggesting low differential stress levels in this zone due tohigh fluid pressure, also facilitating stress rotations. The coseismic stress rotations have similar along-strike extent as themainshock rupture. Postseismic stress rotations tend
to occur in the same locations as the coseismic stressrotations, probably due to the very low remaining
differential stress following the near-complete
coseismic stress drop. The spatial complexity of theobserved stress changes suggests that an analyticalsolution for finding the differential stress from thecoseismic stress rotation may be overly simplistic, andthat modeling of the full spatial distribution of themainshock static stress changes is necessary.

Keywords: Stress, Subduction zone, Fault strength
Graphical abstract

Corresponding author: Jeanne L. Hardebeck, jhardebeck@usgs.gov

FULL PAPER

Open Access

Thermodynamic forward modeling of retrogressive
hydration reactions induced by geofluid infiltration
Tatsu Kuwatani* and Mitsuhiro Toriumi
Earth, Planets and Space 2017, 69:18 DOI:10.1186/s40623-017-0607-5
Received: 18 November 2016, Accepted: 20 January 2017, Published: 30 January 2017

Abstract
We have developed a new methodology for forward analysis of retrogressive hydration (rehydration) reactions by an
improved thermodynamic forward modeling technique based on a differential thermodynamic approach (Gibbs’ method).
Based on natural observations and theoretical considerations, the progress of a rehydration reaction is modeled byincorporating a change in the effective bulk composition on account of the breakdown of the non-equilibrated phase and theamount of water infiltration into the system. Forward analyses of rehydration reactions under greenschist-facies conditions
show that (1) the reaction progress of rehydration is proportional to the external water supply, and (2) the mineralcompositions of equilibrated minerals are mainly controlled by P–T conditions and are similar to those in the global
equilibrium model. Calculated results are in accordance with natural observations of rehydration reactions in greenschistfacies rocks, which supports the validity of the proposed model. The proposedmodel can be used as a basic forward model for various inversion analyses and (a)a Case I: P–T evolution, 500 °C, 1.0 GPa → 300 °C, 0.2 GPa
Water excess
numerical simulations and thus to understand the distribution and behavior of
1
0.5
geofluids.
bar
tar
win
400 °C 500 °C
0.6 GPa 1.0 GPa

Keywords: Rehydration, Forward modeling, Greenschist, Geofluid

XNa.M4

0.75
0.5

0.25

300 °C
0.2 GPa

0.25

act
0

0

500 °C
1.0 GPa*Corresponding author: Tatsu Kuwatani, kuwatani@jamstec.go.jp
5

Quartz

400 °C
0.6 GPa

0

ts
0.5

XAl.T

300 °C
0.2 GPa

Graphical abstract

Epidotehb

0.125 0.25 0.375

Chlorite

Plagioclase

Amphibole, Mantle

AmCrustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales

FULL PAPER

Open Access

Alteration and dehydration of subducting oceanic crust within
subduction zones: implications for décollement step-down
and plate-boundary seismogenesis
Jun Kameda*, Sayako Inoue, Wataru Tanikawa, Asuka Yamaguchi, Yohei Hamada,
Yoshitaka Hashimoto and Gaku Kimura
Earth, Planets and Space 2017, 69:52 DOI:10.1186/s40623-017-0635-1
Received: 20 January 2017, Accepted: 6 April 2017, Published: 18 April 2017

Abstract
The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on themechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts
exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide
insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction
analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage.
These clay mineral reactions may persist to deep crustal levels (~320 °C),
possibly contributing to the bulk dehydration of the basalt and supplying
fluid to plate-boundary fault systems. This dehydration can also cause fluidpressurization at certain horizons within hydrous basalt sequences, eventuallyleading to fracturing and subsequent underplating of upper basement rockinto the overriding accretionary prism. This dehydration-induced breakage ofthe basalt can explain variations in the thickness of accreted basalt fragments
within accretionary prisms as well as the reported geochemical compositionsof mineralized veins associated with exposed basalts in onland locations. This
fracturing of intact basalt can also nucleate seismic rupturing that would
subsequently propagate along seismogenic plate interfaces.

Keywords: Oceanic Crust, Vitrinite Reflectance, Accretionary Prism,
Seismogenic Zone, Nankai TroughGraphical abstract*Corresponding author: Jun Kameda, kameda@sci.hokudai.ac.jp

FULL PAPER

Open AccessWhy do aftershocks occur? Relationship between mainshock
rupture and aftershock sequence based on highly resolved
hypocenter and focal mechanism distributions
Yohei Yukutake* and Yoshihisa Iio
Earth, Planets and Space 2017, 69:68 DOI:10.1186/s40623-017-0650-2
Received: 9 January 2017, Accepted: 2 May 2017, Published: 15 May 2017

Abstract
In order to clarify the origin of aftershocks, we precisely analyze the hypocenters and focal mechanisms of the aftershocks following the 2000
Western Tottori Earthquake, which occurred in the western part of Japan, using data from dense seismic observations. We investigate whether
aftershocks occur on the mainshock fault plane on which coseismic slip occurred or they represent the rupture of fractures surrounding themainshock fault plane. Based on the hypocenter distribution of the aftershocks, the subsurface fault structure of the mainshock is estimatedusing principal component analysis. As a result, we can obtain the detail fault structure composed of 8 best-fit planes. We demonstrate thatthe aftershocks around the mainshock fault are distributed within zones of 1.0–1.5 km in thicknesses, and their focal mechanisms are
significantly diverse. This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault
rather than the rerupture of the mainshock fault. The aftershocks have a much wider zone compared with the exhumed fault zone in fieldobservations, suggesting that many aftershocks occur outside the fault damage zone. We find that most aftershocks except in and around thelarge-slip region are well explained by coseismic stress changes. These results suggestthat the thickness of the aftershock distribution may be controlled by the stressaabbchanges caused by the heterogeneous slip distribution during the mainshock. The
7
7
aftershock is also distributed within a much wider zone than the hypocenter
86
8
6
distribution observed in swarm activity in the geothermal region, which is thought to5
be caused by the migration of hydrothermal fluid. This result implies a difference in
5342generation processes: Stress changes due to the mainshock contribute primarily to the431
occurrence of aftershocks, whereas earthquake swarms in the geothermal region are2
caused by fluid migration within the localized zone.
˚N

35.4

10 km

35.4˚N

˚

35.3

35.3˚

˚

35.2

5˚

1

35.2˚

Keywords: Aftershock, Fault plane of mainshock, Fault damage zone, Hypocenter
distribution, Focal mechanism

133.
4˚

133.
3˚

133.2˚E

133.3˚

133.

133.4˚

2˚E

133.

Fig. 4*Corresponding author: Yohei Yukutake, yukutake@onken.odawara.kanagawa.jp
6Earth, Planets and Space

FULL PAPER

Open AccessFirst report of (U–Th)/He thermochronometric data acrossNortheast Japan Arc: implications for the long-term
inelastic deformation
Shigeru Sueoka*, Takahiro Tagami and Barry P. Kohn
Earth, Planets and Space 2017, 69:79 DOI:10.1186/s40623-017-0661-z
Received: 30 January 2017, Accepted: 29 May 2017, Published: 13 June 2017

Abstract(U–Th)/He thermochronometric analyses were performed across the southern part of the Northeast Japan Arc for reconstructing thelong-term uplift and denudation history in the region. Apatite (U–Th–Sm)/He ages ranged from 64.3 to 1.5 Ma, while zircon (U–Th)/Heages ranged between 39.6 and 11.0 Ma. Apatite (U–Th–Sm)/He ages showed obvious contrast among the morphostructuralprovinces; older ages of 64.3–49.6 Ma were obtained in the Abukuma Mountains on the fore-arc side, whereas younger ages of
11.4–1.5 Ma were determined in the Ou Backbone Range (OBR) along the volcanic front and the Asahi Mountains on the back-arc
side. The age contrasts are basically interpreted to reflect the differences in the uplift and the denudation histories of the provinces
considering the thermal effects of magmatism and timing of the known uplift episodes. Denudation rates were calculated to be<0.1 mm/year in the Abukuma Mountains, ~0.1 to 1 mm/year in the Ou Backbone Range, and ~0.1 to 0.3 mm/year in the Asahi
Mountains. The denudation rates tend to increase from the mountain base to the ridges in the OBR (and the Asahi Mountains). This
relationship shows a contrast with the previous findings in fault-block mountains in the Southwest (SW) Japan Arc, where the highest
denudation rates were estimated near fault(s) along thebase(s). This observation might reflect a difference inmountain uplift mechanisms between the NE and the SW
Japan Arcs and imply that thermochronometric approaches
are useful for constraining uplift and denudation histories atthe scale of an island arc, as well as continental orogens.
However, careful discussion of magmatic thermal effects isrequired.

Keywords: (U–Th)/He thermochronometry, Denudation,
Inelastic deformation, Northeast Japan Arc

Graphical abstract*Corresponding author: Shigeru Sueoka, sueoka.shigeru@jaea.go.jp

FULL PAPER

Open AccessShear strain concentration mechanism in the lower crust below
an intraplate strike-slip fault based on rheological lawsof rocks
Xuelei Zhang* and Takeshi Sagiya
Earth, Planets and Space 2017, 69:82 DOI:10.1186/s40623-017-0668-5
Received: 5 February 2017, Accepted: 8 June 2017, Published: 19 June 2017

Abstract
We conduct a two-dimensional numerical experiment on the lower crust under an intraplate strike-slip fault based on laboratory-derived
power-law rheologies considering the effects of grain size and water. To understand the effects of far-field loading and material properties on
the deformation of the lower crust on a geological time scale, we assume steady fault sliding on the fault in the upper crust and ductile flow
for the lower crust. To avoid the stress singularity, we introduce a yield threshold in the brittle–ductile transition near the down-dip edge ofthe fault. Regarding the physical mechanisms for shear strain concentration in the lower crust, we consider frictional and shear heating, grainsize, and power-law creep. We evaluate the significance of these mechanisms in the formation of the shear zone under an intraplate strike-slip
fault with slow deformation. The results show that in the lower crust, plastic deformation is possible only when the stress or temperature issufficiently high. At a similar stress level, ~100 MPa, dry anorthite begins to undergo plastic deformation at a depth around 28–29 km, which isabout 8 km deeper than wet anorthite. As a result of dynamic recrystallization and grain growth, the grain size in the lower crust may vary
laterally and as a function of depth. A comparison of the results with constant and non-constant grain sizes reveals that the shear zone in thelower crust is created by power-law creep and is maintained by dynamically recrystallized material in the shear zone because grain growthoccurs in a timescale much longer than the recurrence
interval of intraplate earthquakes. Owing to the slow slip
rate, shear and frictional heating have negligible effects
on the deformation of the shear zone. The heatproduction rate depends weakly on the rock rheology;
the maximum temperature increase over 3 Myr is only
about several tens of degrees.

Keywords: Intraplate strike-slip fault, 2-D thermalmechanical fault model, Ductile shear
zone

Graphical abstract*Corresponding author: Xuelei Zhang, zhang@seis.nagoya-u.ac.jp
7Crustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales

FULL PAPER

Open Access

Statistical model selection between elastic and Newtonianviscous matrix models for the microboudinpalaeopiezometer
Tarojiro Matsumura*, Tatsu Kuwatani and Toshiaki Masuda
Earth, Planets and Space 2017, 69:83 DOI:10.1186/s40623-017-0669-4
Received: 27 January 2017, Accepted: 8 June 2017, Published: 22 June 2017

Abstract
We carried out statistical evaluations of two probability density functions for the microboudin palaeopiezometer using the
Akaike information criterion (AIC) and the cross-validation (CV) technique. In terms of the relevant stress-transfer model, thesefunctions are defined as the elastic matrix model and the Newtonian viscous matrix model, respectively. The AIC and CV
techniques enable us to evaluate the relative quality of both models when applied to nine data sets collected from metachert
samples containing tourmaline grains in a quartz matrix, collected from the East Pilbara Terrane, Western Australia. The results
show that the elastic matrix model is the more appropriate probability density function for analysis of fracturing of tourmaline
grains in a quartz matrix. This statistical evaluationshows the validity of the elastic matrix model for the
microboudin palaeopiezometer when analysing suchdata sets.

Keywords: Microboudinage structure, AIC, Cross-validation, Statistical model selection,
Elastic matrix model, Newtonian viscous
matrix model

Graphical abstract*Corresponding author: Tarojiro Matsumura, matsumura-tarojiro@aist.go.jp; f5344007@ipc.shizuoka.ac.jp

FULL PAPER

Open Access

Overpressurized fluids drive microseismic swarm activity
around Mt. Ontake volcano, Japan
Toshiko Terakawa
Earth, Planets and Space 2017, 69:87 DOI:10.1186/s40623-017-0671-x
Received: 24 March 2017, Accepted: 20 June 2017, Published: 29 June 2017

Abstract
Microseismic swarm activity has taken place since 1976 around Mt. Ontake, the second highest stratovolcano in Japan. This
activity is thought to be linked to high pore-fluid pressure in the vicinity of the volcano. We analyzed well-constrained focal
mechanism solutions of microseismicity to re-estimate the 3-D pore-fluidpressure field driving vigorous swarm activity around Mt. Ontake. Pore-fluidpressures were measured by mapping earthquake focal mechanisms on the 3-D
Mohr diagram for the regional stress field with high resolutions of 2–5 km. The
assumption of the reference stress pattern can cause modeling errors inmeasurements of pore-fluid pressure. To remove the effect, we statisticallyevaluated the estimation errors of the regional stress field and included theseerrors in the analysis. We detected an overpressurized fluid reservoir with a peakof about 10–30 MPa in the east flank of Mt. Ontake, where microseismic swarm
activity has been vigorous for the last two decades. The level of pore-fluidpressure was maintained for at least 5 years after 2009. This finding indicatesthat there are some interactions between the intensive swarm activity andoverpressurized fluids: the swarm activity has been driven by overpressurized
fluids, whereas pore-fluid pressures have been suppressed by the swarm
activity.

Keywords: Pore-fluid pressures, Seismic swarm, Stress field, Earthquake focal
mechanisms, Inversion theory

Graphical abstract

Corresponding author: Toshiko Terakawa, terakawa@seis.nagoya-u.ac.jp
8Earth, Planets and Space

FULL PAPER

Open Access

Tensile overpressure compartments on low-angle thrustfaultsRichard H. Sibson
Earth, Planets and Space 2017, 69:113 DOI:10.1186/s40623-017-0699-y
Received: 8 April 2017, Accepted: 31 July 2017, Published: 15 August 2017

Abstract
Hydrothermal extension veins form by hydraulic fracturing under triaxial stress (principal compressive stresses, σ1 > σ2 > σ3)

when the pore-fluid pressure, Pf, exceeds the least compressive stress by the rock’s tensile strength. Such veins form perpendicular to σ3, theirincremental precipitation from hydrothermal fluid often reflected in ‘crack-seal’ textures, demonstrating that the tensile overpressure state,
σ3′ = (σ3 − Pf ) < 0, was repeatedly met. Systematic arrays of extension veins develop locally in both sub-metamorphic and metamorphic
assemblages defining tensile overpressure compartments where at some time Pf > σ3. In compressional regimes (σv = σ3), subhorizontal
extension veins may develop over vertical intervals <1 km or so below low-permeability sealing horizons with tensile strengths
10 < To < 20 MPa. This is borne out by natural vein arrays. For a low-angle thrust, the vertical interval where the tensile overpressure stateobtains may continue down-dip over distances of several kilometres in some instances.
Shear Fractures (Faults)
Extension Fractures
The overpressure condition for hydraulic fracturing is comparable to that needed for
3
3
frictional reshear of a thrust fault lying close to the maximum compression, σ1. Under
these circumstances, especially where the shear zone material has varying competence
1
1(tensile strength), affecting the failure mode, dilatant fault–fracture mesh structures
may develop throughout a tabular rock volume. Evidence for the existence of fault–()/ < 4()/ > 5.66
fracture meshes around low-angle thrusts comes from exhumed ancient structuresand from active structures. In the case of megathrust ruptures along subduction
Extensional-Shear Fracturesinterfaces, force balance analyses, lack of evidence for shear heating, and evidence of3
total shear stress release during earthquakes suggest the interfaces are extremely
weak (τ < 40 MPa), consistent with weakening by near-lithostatically overpressured
1
fluids. Portions of the subduction interface, especially towards the down-dip)/ < 5.66
4 < (termination of the seismogenic megathrust, are prone to episodes of slow-slip,
non-volcanic tremor, low-frequency earthquakes, very-low-frequency earthquakes,
etc., attributable to the activation of tabular fault–fracture meshes at low σ3′ aroundthe thrust interface. Containment of near-lithostatic overpressures in such settings is
Fig. 1precarious, fluid loss curtailing mesh activity.
1

3

1

1

3

3

Corresponding author: Richard H. Sibson, rick.sibson@otago.ac.nz

FULL PAPER

Open Access

Dynamic rupture propagation on geometrically complex fault
with along-strike variation of fault maturity: insights fromthe 2014 Northern Nagano earthquake
Ryosuke Ando*, Kazutoshi Imanishi, Yannis Panayotopoulos and Tomokazu Kobayashi
Earth, Planets and Space 2017, 69:130 DOI:10.1186/s40623-017-0715-2
Received: 4 April 2017, Accepted: 5 September 2017, Published: 18 September 2017

Abstract
Understanding the effect of the complex fault geometry on the dynamic rupture process and discriminating it from thecomplexity originating from the rheological properties of faults, is an essential subject in earthquake science. The 2014
Northern Nagano earthquake, which occurred near the end zone of a major active fault system, provided unique observations
that enabled us to investigate both the detailed geometrical fault structure and surface
deformation patterns as well as the temporal sequence led up from a prominent
foreshock activity. We first develop a geometrical fault model with a substantial variation
along strike, and a model for the regional stress field, which is well constrained by theobservations. This significant along-strike variation in fault geometry probably reflects thedifference of fault maturity at the end zone of the complex fault system. We used thismodel in order to conduct a set of dynamic rupture simulations using the highly efficientspatiotemporal boundary integral equation method. Based on our simulations, we showthat the observed surface deformation can be reasonably explained as the effect of the
non-planar fault geometry with a number of branch faults and bends.

Keywords: 2014 Northern Nagano earthquake, Fault geometry, Dynamic rupture, InSAR,
Fast domain partitioning BIEM

Graphical abstract*Corresponding author: Ryosuke Ando, ando@eps.s.u-tokyo.ac.jp
9Crustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales

FULL PAPER

Open Access

Which is heterogeneous, stress or strength? An estimationfrom high-density seismic observations
Yoshihisa Iio*, Itaru Yoneda, Masayo Sawada, Tsutomu Miura, Hiroshi Katao, Yoichiro Takada,
Kentaro Omura and Shigeki Horiuchi
Earth, Planets and Space 2017, 69:144 DOI:10.1186/s40623-017-0730-3
Received: 22 April 2017, Accepted: 11 October 2017, Published: 20 October 2017

Abstract
Using data from high-density seismic observation networks installed in the western Nagano Prefecture region in Japan, we
precisely determined focal mechanisms and estimated the high-resolution stress field at a scale of 1 km. Almost all differences
between observed and calculated slip directions (misfit) were smaller than the errors in focal mechanisms at grid points awayfrom the mainshock fault. This finding clearly indicates that the estimated uniform stress suitably explains focal mechanisms
in each subregion apart from the mainshock fault. Misfits are relatively large at grid points near the mainshock fault, but manyof these misfits are smaller than the errors in focal mechanisms, and stress is regarded as uniform for a greater portion withineach subregion. However, we found that focal mechanisms and P-axes varied widely and differed from each other for a short
focal distance of 100 m. These results
clearly show that stress can be regarded
as uniform, but that strength isheterogeneous.

Keywords: Stress, Heterogeneity, Fault
strength, Stress inversion,
Focal mechanism

Graphical abstract*Corresponding author: Yoshihisa Iio, iio@rcep.dpri.kyoto-u.ac.jp

FULL PAPER

Open Access

Strain localization and fabric development in polycrystalline
anorthite + melt by water diffusion in an axial deformation
experimentJun-ichi Fukuda*, Jun Muto and Hiroyuki Nagahama
Earth, Planets and Space 2018, 70:3 DOI:10.1186/s40623-017-0776-2
Received: 31 March 2017, Accepted: 26 December 2017, Published: 4 January 2018

Abstract
We performed two axial deformation experiments on synthetic polycrystalline anorthite samples with a grain size of ~ 3 μm and 5 vol%Si–Al-rich glass at 900 °C, a confining pressure of 1.0 GPa, and a strain rate of 10−4.8 s−1. One sample was deformed as-is (dry); in the othersample, two half-cut samples (two cores) with 0.15 wt% water at the boundary were put together in the apparatus. The mechanical data forboth samples were essentially identical with a yield strength of ~ 700 MPa and strain weakening of ~ 500 MPa by 20% strain. The dry sample
appears to have been deformed by distributed fracturing. Meanwhile, the water-added sample shows plastic strain localization in addition to
fracturing and reaction products composed of zoisite grains and SiO2 materials along the boundary between the two sample cores. Infrared
spectra of the water-added sample showed dominant water bands of zoisite. The maximum water content was 1500 wt ppm H2O at thetwo-core boundary, which is the same as the added amount. The water contents gradually decreased from the boundaries to the sample
interior, and the gradient fitted well with the solution of the one-dimensional diffusion equation. The determined diffusion coefficient was
7.4 × 10−13 m2/s, which agrees with previous data for the grain boundary diffusion of water. The anorthite grains in the water-added sample
showed no crystallographic preferred orientation. Textural observations and water diffusion indicatethat water promotes the plastic deformation of polycrystalline anorthite by grain-size-sensitive creep aswell as simultaneous reactions. We calculated the strain rate evolution controlled by water diffusion in
10feldspar aggregates surrounded by a water source. We assumed water diffusion in a dry rock mass with
10variable sizes. Diffused water weakens a rock mass with time under compressive stress. The calculated
10strain rate decreased from 10−10 to 10−15 s−1 with an increase in the rock mass size to which water is
10supplied from < 1 m to 1 km and an increase in the time of water diffusion from < 1 to ~ 10,000 years.
10- 10 This indicates a decrease in the strain rate in a rock mass with increasing deformation via water
10Ma10diffusion.
ss
(/s)

10

rate
Strain

8

12

3

10

4

14

10

16

Plastic deformation, Reaction*Corresponding author: Jun-ichi Fukuda, jfukuda@eps.s.u‑tokyo.ac.jp
10

10

)

10 3

Graphical abstract

ea
r

(y

e

10
0

10 2

m

(m

Ti

1

ale

10
-1

Keywords: Anhydrous polycrystalline anorthite, Water diffusion, Griggs-type deformation apparatus,

10
1

0

sc

s)

2

1

Earth, Planets and Space

FULL PAPER

Open Access

In-situ permeability of fault zones estimated by hydraulictests and continuous groundwater-pressure observations
Norio Matsumoto* and Norio Shigematsu
Earth, Planets and Space 2018, 70:13 DOI:10.1186/s40623-017-0765-5
Received: 31 March 2017, Accepted: 19 December 2017, Published: 25 January 2018

AbstractcIn-situ permeability of the Median Tectonic Line (MTL) fault zone in Mie Prefecture, southwest
Japan, was estimated using hydraulic tests and groundwater pressure observations in twoboreholes. The screen depths in Holes 1 and 2 are located, respectively, in a major strand of the
MTL fault zone within the Sambagawa metamorphic rocks and a branching fault developed in the
hanging wall of the MTL within the Ryoke mylonite. The estimated permeability at Hole 1 ranges
from 5.3 × 10−17 to 5.0 × 10−16 m2, and that at Hole 2 ranges from 4.4 × 10−16 to 1.5 × 10−15 m2. We
also measured the permeability of the protolith close to the screened depth of Hole 1 (3.4 × 10−19
and 3.7 × 10−19 m2) and Hole 2 (3.1 × 10−19 and 6.2 × 10−19 m2). The permeability of the fault zone
was found to be more than 100 and 700 times higher than the protolith permeability at Holes 1
and 2, respectively. The permeability data for Holes 1 and 2 are consistent with previously reported
permeability data for samples from an MTL outcrop. The permeability observed in this study
reflects the complex fault zone permeability structure of the MTL fault zone.

Keywords: Fault zone permeability, Slug test, Pumping test, Permeability of drillcore sample,
Median Tectonic Line

Fig. 1*Corresponding author: Norio Matsumoto, n.matsumoto@aist.go.jp

FULL PAPER

Open AccessComplex microseismic activity and depth-dependent stressfield changes in Wakayama, southwestern Japan
Sumire Maeda*, Toru Matsuzawa, Shinji Toda, Keisuke Yoshida and Hiroshi Katao
Earth, Planets and Space 2018, 70:21 DOI:10.1186/s40623-018-0788-6
Received: 25 March 2017, Accepted: 24 January 2018, Published: 7 February 2018

Abstract
We examined the spatial relationship between seismicity and upper crustal structure in the Wakayama region, northwestern
Kii Peninsula, Japan, by investigating microearthquake focal mechanisms and the local stress field. The focal mechanisms of
most events studied fall into three categories: (1) normal faulting with N–S-oriented T-axes mainly occurring at shallow
depths, (2) reverse faulting with E–W-oriented P-axes dominating at intermediate depths, and (3) strike-slip faulting with
N–S-oriented T-axes and E–W-oriented P-axes mainly seen at greater depths. The stress field varies with depth: the shallow
part is characterized by a strike-slip-type stress regime with N–S tension and E–W compression, while the deep part ischaracterized by an E–W compressional stress regime consistent with reverse faulting. The depth-dependent stress regime
can be explained by thermal stress caused by a heat source, as expected from geophysical observations. Geologic faults,
acting as weak planes, might contributebto generate shallow normal fault-type and a
deeper strike-slip fault-type
microearthquakes.

Keywords: Microearthquake, Stress field,

Heterogeneous structure,
Fluid, Heat source, Geologicalstructure
Graphical abstract*Corresponding author: Sumire Maeda, maeda.sumire.q6@dc.tohoku.ac.jp
11Crustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales

FULL PAPER

Open AccessInterseismic crustal deformation in and around the Atotsugawa
fault system, central Japan, detected by InSAR and GNSS
Youichiro Takada*, Takeshi Sagiya and Takuya Nishimura
Earth, Planets and Space 2018, 70:32 DOI:10.1186/s40623-018-0801-0
Received: 31 March 2017, Accepted: 7 February 2018, Published: 16 February 2018

Abstract
The Atotsugawa fault system is one of the best-known active faults in Japan.
However, revealing the interseismic velocity field in and around the Atotsugawa
fault system with high spatial resolution is challenging because of dense
vegetation, steep topography, and heavy snowfall in winter. To overcome these
difficulties, we combined ALOS/PALSAR data and GNSS data from our original
stations in addition to the nationwide station network (GEONET). First, weremoved the height-dependent phase change in each interferogram using a
digital elevation model. Next, we removed the long-wavelength phase trendusing the GNSS velocity field. Finally, we applied an InSAR time-series analysis,known as small baseline subset analysis (SBAS), to all the correctedinterferograms. The resultant mean velocity field shows a remarkable phasegradient around the Atostugawa fault system. We found a sharp velocity
gradient across the Ushikubi fault, a major strand of the Atotsugawa faults
system, rather than the main trace of the Atotsugawa fault. Using InSAR, wefound that the interseismic deformation inside the strain concentration zone isspatially heterogeneous and different from what we expect from the fault traces.

136.9˚E137˚E137.1˚E

137.2˚E

137.3˚E

137.4˚E

36.7˚NA’

36.6˚N

LOS [mm/yr]

F

5

UK

36.5˚N

X

4

F
MS
ATF

36.4˚N32

1

0
36.3˚N−1

Y−2

36.2˚NA−3−4

Keywords: InSAR, GNSS, Atotsugawa fault, Interseismic velocity−5Graphical abstract*Corresponding author: Youichiro Takada, takaday@sci.hokudai.ac.jp

FULL PAPER

Open Access

Normal-faulting stress state associated with low differential
stress in an overriding plate in northeast Japan prior
to the 2011 Mw 9.0 Tohoku earthquake
Makoto Otsubo*, Ayumu Miyakawa and Kazutoshi Imanishi
Earth, Planets and Space 2018, 70:51 DOI:10.1186/s40623-018-0813-9
Received: 27 January 2017, Accepted: 7 March 2018, Published: 26 March 2018

Abstract
Spatial and temporal variations in inland crustal stress prior to the 2011 Mw 9.0 Tohoku earthquake are investigated using focal
mechanism solutions for shallow seismicity in Iwaki City, Japan. The multiple inverse method of stress tensor inversion detected two
normal-faulting stress states that dominate in different
regions. The stress field around Iwaki City changed from a
NNW–SSE-trending triaxial extensional stress (stressregime A) to a NW–SE-trending axial tension (stressregime B) between 2005 and 2008. These stress changes
may be the result of accumulated extensional stressassociated with co- and post-seismic deformation due tothe M7 class earthquakes. In this study we suggest thatthe stress state around Iwaki City prior to the 2011 Tohoku
earthquake may have been extensional with a low
differential stress. High pore pressure is required to cause
earthquakes under such small differential stresses.

Keywords: Crustal stress, 2011 Tohoku earthquake,
Deformation, Stress tensor inversion,
Differential stress

Graphical abstract*Corresponding author: Makoto Otsubo, otsubo-m@aist.go.jp
12Earth, Planets and Space

EXPRESS LETTER

Open Access

Anelastic properties beneath the Niigata–Kobe Tectonic
Zone, Japan
Junichi Nakajima* and Toru Matsuzawa
Earth, Planets and Space 2017, 69:33 DOI:10.1186/s40623-017-0619-1
Received: 11 January 2017, Accepted: 13 February 2017, Published: 22 February 2017

Abstract
We estimate the three-dimensional (3D) P-wave attenuation structure beneath the Niigata–Kobe Tectonic Zone (NKTZ),
central Japan, using high-quality waveform data from a large number of stations. The obtained results confirm thesegmentation of the NKTZ into three regions, as suggested by 3D seismic velocity models, and reveal characteristic structures
related to surface deformation, shallow subduction of the Philippine Sea slab, and magmatism. The lower crust beneath the
NKTZ west of the Itoigawa–Shizuoka Tectonic Line (ISTL) is overall characterized by distinct high attenuation, whereas theupper crust shows marked high attenuation to the east of the ISTL. Differences in the depths of anelastically weakened partsof the crust probably result in a first-order spatial variation in surface deformation, forming wide (width of ~100 km) andnarrow (width of 25–40 km) deformation zones on the western and eastern sides of the ISTL, respectively. Many M ≥ 6.5
earthquakes occur in the upper crust where seismic attenuation in the underlying lower crust varies sharply, suggesting thatspatial variations in rates of anelastic deformation in the lower crust result in stress concentration in the overlying brittle crust.
We interpret a moderate- to low-attenuation
zone located in the lower crust at thenortheast of Biwa Lake to reflect lowtemperature conditions that are developed
locally as a result of shallow subduction of
the cold Philippine Sea slab.

Keywords: Seismic attenuation, Strain,

Surface deformation, Slab, Grain
size

Graphical abstract*Corresponding author: Junichi Nakajima, nakajima@geo.titech.ac.jp

EXPRESS LETTER

Open Access

Spatial variation in coda Q in the northeastern part of
Niigata–Kobe Tectonic Zone, central Japan: implication of
the cause of a high strain rate zone
Masanobu Dojo and Yoshihiro Hiramatsu*
Earth, Planets and Space 2017, 69:76 DOI:10.1186/s40623-017-0663-x
Received: 26 January 2017, Accepted: 29 May 2017, Published: 5 June 2017

Abstract
We have analyzed the spatial variation in coda Q in the northeastern part of a high strain rate zone, the Niigata–Kobe Tectonic
Zone (NKTZ), to investigate the cause of the high strain rate by correlating coda Q with the differential strain rate and theS-wave velocity. The spatial distributions of coda Q in the 2–4 and 4–8 Hz frequency bands are found to be negatively
correlated with the differential strain rate. Coda Q in the 2–4 Hz frequency band is correlated spatially with the S-wave velocity
at a 25 km depth, as has been reported previously. We also find a positive correlation between the perturbation of the S-wave
velocity at a 10 km depth and coda Q in the 4–8 Hz frequency band, implying that the spatial distribution of coda Q in thisfrequency band is mainly attributed to the heterogeneity of the upper crust. This feature is different from that of the central
part of the NKTZ reported previously, whichindicates a difference in the cause of the high strain
rate. Therefore, we suggest that the highdeformation rate in the upper crust, which ischaracterized by a thick sediment basin, as well asthat in the ductile lower crust, contributes to thegeneration process of the high strain rate on thesurface in the northeastern part of the NKTZ.

Keywords: Heterogeneity, Differential strain rate,
S-wave velocity, Ductile deformation

Graphical abstract*Corresponding author: Yoshihiro Hiramatsu, yoshizo@staff.kanazawa-u.ac.jp
13Crustal Dynamics: Unified Understanding of Geodynamics Processes at Different Time and Length Scales

EXPRESS LETTER

Open Access

San-in shear zone in southwest Japan, revealed by
GNSS observations
Takuya Nishimura* and Youichiro Takada
Earth, Planets and Space 2017, 69:85 DOI:10.1186/s40623-017-0673-8
Received: 30 March 2017, Accepted: 20 June 2017, Published: 27 June 2017

Abstract
A right-lateral shear zone in the San-in region, southwest Japan, has been proposed by previous geological and seismologicalstudies. It locates 350 km north of the Nankai Trough, that is, the main plate boundary between the subducting Philippine Sea
and overriding Amurian plates and presumably accommodates a part of the relative plate motion. We present a geodetic
evidence of the proposed shear zone using GNSS velocity data. Distinct shear deformation is identified only between ~132.5°E
and ~135°E along a coastline which is a part of the proposed shear zone, and we propose to call the geodetically identified
shear zone as the San-in shear zone (SSZ). The SSZ is a concentrated deformation zone with a width of ~50 km and can bemodeled by a deep creep on a vertical strike slip fault with a creep rate of ~5 mm/year. There are some active faults paralleland oblique to the overall trend of the SSZ, but no single active fault coincides with the SSZ. Lineaments of microseismicity
and source faults of large earthquakes are almost oriented in NNW–SSE in the SSZ and oblique to the overall trend of the SSZ.
They are interpreted as conjugate Riedel shears. Based on
35.5˚
these geodetic, seismological, and geomorphological
σ1 San-in Shear Zone
5mm/yrσ1
R´N
observations, we suggest that the SSZ is a developing
1927
R
1943
D
and young shear zone in a geological time scale.
1925
2000

35˚

2016

Shear zone

Keywords: San-in shear zone (SSZ), Strain concentration

Ref.

135.5˚

135˚134.5˚134˚133.5˚

133˚

34.5˚

132.5˚zone, GNSS, Southwest Japan

σ1

Graphical abstract*Corresponding author: Takuya Nishimura, nishimura.takuya.4s@kyoto-u.ac.jp

EXPRESS LETTER

Open Access

Sintering polycrystalline olivine and polycrystalline
clinopyroxene containing trace amount of graphitefrom natural crystals
Yumiko Tsubokawa* and Masahiro Ishikawa
Earth, Planets and Space 2017, 69:128 DOI:10.1186/s40623-017-0717-0
Received: 9 April 2017, Accepted: 7 September 2017, Published: 16 September 2017

Abstract
Graphite-bearing polycrystalline olivine and polycrystalline clinopyroxene with submicron to micron grain size weresuccessfully sintered from a single crystal of naturally occurring olivine (Fo88–92Fa12–8: Mg1.76–1.84Fe0.16–0.24SiO4) and a single crystalof naturally occurring clinopyroxene (Di99Hed1: Ca0.92Na0.07Mn0.01Mg0.93Fe0.01Al0.06Si2O6). The
milled powders of both these crystals were sintered under argon gas flow at temperatures
ranging from 1130 to 1350 °C for 2 h. As the sintering temperature increased, the averagegrain size of olivine increased from 0.2 to 1.4 µm and that of clinopyroxene increased from
0.1 to 2.4 µm. The porosity of sintered samples remained at an almost-constant volume of
2–5% for olivine and 3–4% for clinopyroxene. The samples sintered from powders milledwith ethanol exhibited trace amount of graphite, identified via Raman spectroscopy
analysis. As the sintering temperature increased, the intensity of the graphite Raman peakdecreased, compared with both olivine and clinopyroxene peaks. The carbon content of
the sintered samples was estimated to be a few hundred ppm. The in-plane size (La) ofgraphite in the sintered olivine was estimated to be <15 nm. Our experiments demonstrate
new possibilities for preparing graphite-bearing silicate-mantle mineral rocks, and thismethod might be useful in understanding the influence of the physical properties ofgraphite on grain-size-sensitive rheology or the seismic velocity of the Earth’s mantle.

Keywords: Graphite, Olivine, Clinopyroxene, SubmicronFig. 2*Corresponding author: Yumiko Tsubokawa, tsubokawa-yumiko-nd@ynu.jp
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Contents
Crustal dynamics: unified understanding of geodynamic processes at different time and length scales. . . . . . . . . . . . . . . . . . . . . . . . . . Yoshihisa Iio, Richard H. Sibson, Toru Takeshita, Takeshi Sagiya, Bunichiro Shibazaki and Junichi Nakajima
The spatial distribution of earthquake stress rotations following large subduction zone earthquakes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jeanne L. Hardebeck
Thermodynamic forward modeling of retrogressive hydration reactions induced by geofluid infiltration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tatsu Kuwatani and Mitsuhiro Toriumi
Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement
step-down and plate-boundary seismogenesis. . . . . . . . . . . . . . . Jun Kameda, Sayako Inoue, Wataru Tanikawa, Asuka Yamaguchi,
Yohei Hamada, Yoshitaka Hashimoto and Gaku Kimura

Why do aftershocks occur? Relationship between mainshock rupture and aftershock sequence based on highly
resolved hypocenter and focal mechanism distributions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yohei Yukutake and Yoshihisa Iio
First report of (U–Th)/He thermochronometric data across Northeast Japan Arc: implications for the long-term inelastic
deformation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shigeru Sueoka, Takahiro Tagami and Barry P. Kohn
Shear strain concentration mechanism in the lower crust below an intraplate strike-slip fault based on rheological lawsof rocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xuelei Zhang and Takeshi Sagiya
Statistical model selection between elastic and Newtonian viscous matrix models for the microboudinpalaeopiezometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tarojiro Matsumura, Tatsu Kuwatani and Toshiaki Masuda
Overpressurized fluids drive microseismic swarm activity around Mt. Ontake volcano, Japan. . . . . . . . . . . . . . Toshiko Terakawa
Tensile overpressure compartments on low-angle thrust faults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Richard H. Sibson
Dynamic rupture propagation on geometrically complex fault with along-strike variation of fault maturity: insightsfrom the 2014 Northern Nagano earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryosuke Ando, Kazutoshi Imanishi, Yannis Panayotopoulos and Tomokazu Kobayashi
Which is heterogeneous, stress or strength? An estimation from high-density seismic observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yoshihisa Iio, Itaru Yoneda, Masayo Sawada, Tsutomu Miura, Hiroshi Katao,
Yoichiro Takada, Kentaro Omura and Shigeki Horiuchi

Strain localization and fabric development in polycrystalline anorthite + melt by water diffusion in an axial
deformation experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jun-ichi Fukuda, Jun Muto and Hiroyuki Nagahama
In-situ permeability of fault zones estimated by hydraulic tests and continuous groundwater-pressure observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Norio Matsumoto and Norio Shigematsu
Complex microseismic activity and depth-dependent stress field changes in Wakayama, southwestern Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sumire Maeda, Toru Matsuzawa, Shinji Toda, Keisuke Yoshida and Hiroshi Katao
Interseismic crustal deformation in and around the Atotsugawa fault system, central Japan, detected by InSAR and
GNSS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Youichiro Takada, Takeshi Sagiya and Takuya Nishimura
Normal-faulting stress state associated with low differential stress in an overriding plate in northeast Japan prior to
the 2011 Mw 9.0 Tohoku earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Makoto Otsubo, Ayumu Miyakawa and Kazutoshi Imanishi
Anelastic properties beneath the Niigata–Kobe Tectonic Zone, Japan. . . . . . . . . . . . . . . . . . . Junichi Nakajima and Toru Matsuzawa
Spatial variation in coda Q in the northeastern part of Niigata–Kobe Tectonic Zone, central Japan: implication of
the cause of a high strain rate zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masanobu Dojo and Yoshihiro Hiramatsu
San-in shear zone in southwest Japan, revealed by GNSS observations. . . . . . . . . . . . . . . . Takuya Nishimura and Youichiro Takada
Sintering polycrystalline olivine and polycrystalline clinopyroxene containing trace amount of graphite from natural
crystals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yumiko Tsubokawa and Masahiro Ishikawa

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Cover image: Nishimura, T. et al., Earth, Planets and Space 2017, 69:85 DOI:10.1186/s40623-017-0673-8